High efficiency switching drive for a resonate power transformer

ABSTRACT

A high efficiency switching drive for a resonate power transformer having the ability to maintain a constant output voltage over a wide range of source voltage variation while maintaining good power conversion efficiency is provided. A transistorized class-C oscillator has means to saturate the transistors during conduction time to allow the transistor voltage to remain small as opposed to a current source as in the conventional class-C oscillator.

BACKGROUND OF THE INVENTION

In a portable electrical instrument or similar apparatus high-efficiencyoperation is axiomatic, especially if such instrument or apparatusderives required power from a DC voltage source such as a battery.Additionally, in electrical instruments or similar apparatus, it isessential to high efficiency operation to maintain a constant outputvoltage over a wide range of source voltage variation.

Prior art portable electrical instruments as well as standard electricalinstruments have included circuits to improve the efficiency ofoperation. For example, a discussion concerning class-C oscillators andthe difficulty of combining high efficiency within a satisfactory degreewas presented at the International Convention on Transistors andAssociated Semiconductor Devices, May 25, 1959; see The Institution ofElectrical Engineers Paper No. 2978E, February 1960, "TRANSISTORSINEWAVE LC OSCILLATORS" by P. J. Baxandall. However, omitted therefromis the ability to maintain a constant output voltage over a wide rangeof source voltage variation while maintaining good power conversionefficiency.

SUMMARY OF THE INVENTION

The subject invention overcomes the disadvantages of the prior art inthat a high efficiency switching drive for a resonate power transformeris provided which has the ability to maintain a constant output voltageover a wide range of source voltage variation while maintaining goodpower conversion efficiency. Basically, a transistorized class-Coscillator (i.e., blocking oscillator) has means to saturate thetransistors during conduction time. This allows a high Q transformer tobe driven at its natural frequency while maintaining the high powerefficiency of a switching converter over a broad range of input voltageand output load variation with a constant output voltage.

It is therefore an object of the present invention to provide a highefficiency switching drive for a resonate power transformer to overcomethe disadvantages of the prior art.

It is another object of the present invention to provide an improvedDC-to-DC converter having high efficiency.

It is a further object of the present invention to provide a class-Coscillator having high efficiency.

The foregoing and numerous other objects, advantages, and inherentfunctions of the present invention will become apparent as the same ismore fully understood from the following description, which describesthe invention; it is to be understood, however, that the variousembodiments are not intended to be exhausting nor limiting of theinvention but are given for purposes of illustration in order thatothers skilled in the art may fully understand the invention andprinciples thereof and the manner of applying it in practical use sothat they may modify it in various forms, each as may be best suited tothe conditions of the particular use.

DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is the basic circuit according to the present invention;

FIG. 2 is the timing diagram for the basic circuit according to FIG. 1;

FIG. 3 is a hybrid block and circuit diagram of a system according topresent invention; and

FIG. 4 is an alternative embodiment according to the present invention.

DESCRIPTION OF THE INVENTION

Referring now to the drawings and in particular to FIG. 1, there isshown the basic circuit of the high efficiency switching drive for aresonate power transformer according to the present invention. Seriallydisposed between an input terminal 10, to which may be applied a DCvoltage or a rectified AC voltage, and a source of reference potential,say, ground is a first winding 12 of a resonate transformer 14, a firstwinding 16 of a second transformer 18, and the collector-emitterjunction of a first transistor 20. Serially disposed between inputterminal 10 and the junction between winding 16 and first transistor 20is a nonsymmetrical conducting device such as the diode 22 and thecollector-emitter junction of a second transistor 24. The base of secondtransistor 24 is also connected to such junction via a nonsymmetricalconducting device such as the diode 26 and to the junction betweenwindings 12, 16 via an impedance means such as a resistor 28 whereas thebase of first transistor 20 is connected to a current source 30 viaseries connected second winding 32 of second transformer 18 and secondwinding 34 of first transformer 14. A capacitor 36 is connected betweenthe source of reference potential and the junction between currentsource 30 and second winding 34 of transformer 14. Current source 30 isof the type whereby its output is dependent upon a control signaldeveloped, say, by a conventional regulator circuit. An additional inputterminal 10' is connected to the source of reference potential forobvious reasons. The output from the basic circuit is taken across thirdwinding 38 of transformer 14. A plurality of polarity indicators 40 arealso shown on the diagram.

The basic circuit timing (see FIG. 2) is dependent upon the naturalperiod of transformer 14 and the rate at which capacitor 36 is chargedby the regulating circuit. When the sum of the voltage across capacitor36 and the voltage across second winding 34 of transformer 14 exceedsapproximately 0.6 volts, transistor 20 begins to conduct. (It is assumedV supply is applied between terminals 10 and 10'). The conductioncurrent I_(L) causes an induced voltage into secondary winding 32 oftransformer 18 to enhance the turn on of transistor 20. Thus, transistor20 saturates due to transformer action and becomes a switch rather thana conventional current source in a conventional class-C oscillator.Simultaneously the voltage V₁ approaches 0 volts in a sinusoidal mannerwhile transformer action provides power to the secondary load (notshown) connected across winding 38 of transformer 14 whereas winding 16of transformer 18 stores energy due to the changing current flowingtherethrough.

At an instant when V₁ is at its most negative position and begins tomove positive, the voltage V₃ and the voltage across windings 34 and 32are such that transistor 20 tends to become reversed biased.Instantaneously, the collector voltage V₂ of transistor 20 movespositive and due to action of transformer 18 quickly reverse biasestransistor 20 completely. Transistor 24 conducts transferring energystored in winding 16 to transformer 14. In essence, when transistor 24is switched on, winding 16 is effectively shunted across the winding 12and energy stored in the winding 16 is utilized to drive the transformer14. Thus, since existing energy is used to drive the system, the systemis more efficient.

As can be discerned from the above, the subject invention is a switchingscheme and as such, the semiconductor devices must be fast.Additionally, since transistors 20 and 24 see the peak current pulse,the devices must have betas commensurate at the maximum currentexpected.

Shown in FIG. 3 is a system employing the present invention. Forconvenience, reference numerals common to all embodiments will be usedto show equivalence. FIG. 3 is a DC to DC converter made in accordancewith the present invention. A load 62 may be, for example, a utilizationcircuit such as the cathode-ray-tube. The voltage developed acrosssecondary winding 38A is applied to the load 62 via a conventionalfilter and rectifier means 60. Additionally, feedback to control thesystem is obtained via the regulator 64 (includes a current source 30).Diode 65 has been added to eliminate power losses of resistor 28A duringnonconduction of transistor 24A. Circuit operation is identical to thatgiven for the basic circuit.

Shown in FIG. 4 is another embodiment according to the presentinvention. An additional transistor 78 has been disposed across theemitter base junction of transistor 20B. The base of transistor 78 isconnected to current source 30B via series connected diode 70, 72 poledso as to steer current into the base of transistor 78 in accordance withthe charge on a capacitor 80 disposed between such diodes and winding38B of transformer 38B. Capacitor 80 can be selectively connected towinding 14B. A resistor 76 has also been added to properly operatetransistor 78 whereas the capacitor 74 reduces noise generated by thefast switching action of transistor 20B. In this alternative embodiment,the charge on capacitor 80 is directly proportional to the voltageV_(IB). Thus, the exact point at which switching of transistor 20B isaccomplished. Basically, the voltage across winding 38B is sampledwhich, in turn, provides current shifted in phase from the voltage by90° to turn transistor 78 on. When transistor 78 conducts, transistor20B stops conducting as previously explained.

While there has been shown and described the preferred embodiments ofthe present invention, it will be apparent to those skilled in the artthat many changes and modifications may be made without departing fromthe invention in its broader aspects. For example, any transformer can,with the addition of conventional components be made equivalent to aresonate transformer. Therefore, the appended claims are intended tocover all such changes and modifications that fall within the truespirit and scope of the invention.

The invention is claimed in accordance with the following:
 1. A circuitfor improving the efficiency of an energy conversion apparatus,comprising:a resonate transformer having primary windings and secondarywindings electromagnetically coupled thereto for converting the energy;means, coupled to said primary windings, for storing energy .[.developedby said primary windings.].; first switch means coupled to said meansfor storing energy, said first switch means having a first condition anda second condition; means for impressing a potential of an externalenergy source across said primary windings and said means for storingenergy in said first condition of said first switch means; and secondswitch means coupled to said primary windings and operatively disposedacross said means for storing energy in said second condition of saidfirst switch means for impressing energy stored in said first conditionto said primary windings thereby improving efficiency of the apparatus.2. The circuit according to claim 1 further comprising feedback meansoperatively disposed between said second windings and said first switchmeans for controlling said first switch means.
 3. The circuit accordingto claim 2 wherein said feedback means determines said first and secondconditions of said first switch means.
 4. The circuit according to claim1 wherein said first switch means defines a blocking oscillatorcontaining an oscillating transistor.
 5. The circuit according to claim1 wherein said means for storing energy defines a transformer windingconnected to said primary winding.
 6. The circuit according to claim 1wherein said second switch means defines an amplifier means including aswitching transistor.